More precisely, the aim of the invention is to develop a method of treating CO2 coming from an oxy-combustion process (i.e. combustion with pure oxygen or with a gas leaner in nitrogen than air).
Now, fossil fuel and/or biomass combustion gases or waste incineration gases or gases coming from glass furnaces predominantly contain heavy metals, such as mercury, organic pollutants and compounds of the SOx and NOx type.
The danger of mercury pollution is known, since mercury is a toxic element. In humans, exposure to mercury may lead to neurological and growth disorders.
Mercury emissions may remain in the atmosphere from a few hours to several years.
Furthermore, it is essential to provide a mercury extraction method for the treatment of combustion gases.
It is known that certain metals, for example gold, silver and copper, form amalgams with mercury and that this property is used for assaying mercury.
However, the extraction of mercury by these metals is not used on an industrial scale because of the hourly volume to be treated.
Another possibility is to wash coal before combustion. However, the washing operations make it possible to remove only between 0 and 60% of the mercury.
Other solutions, such as electrostatic precipitators or filter membrane separators, have been tried out.
However, no trial has been conclusive.
Moreover, U.S. Pat. No. 4,094,777 describes a method for capturing mercury in gas or liquid phase by fixing the mercury on an adsorbent mass comprising copper sulfide and optionally silver sulfide deposited on a support based on silica and/or alumina. The method is a fixed bed method that can be used at temperatures from −50° C. to 200° C. and at pressures up to 200 bar. It states that this method is applicable to the demercuration of natural gas or electrolytic hydrogen. The example given relates to natural gas containing methane, hydrocarbons up to C5, CO2 and nitrogen.
It has also been disclosed that regeneration may take place with air at a temperature of 200° C. and above, and that it may be necessary to resulfurize the material.
U.S. Pat. No. 4,909,926 describes a method for capturing mercury in a hydrocarbon stream. The materials cited comprise Ag and CuS, the supports being alumina or SiO2.
When an alumina support is used, regeneration takes place with a gas containing oxygen, the oxygen being purged before reutilization. This document mentions that this purge must take place at high temperature in order to avoid oxide formation. Finally, it mentions that the method may be applied in the presence of hydrocarbons, hydrogen, H2S, N2, H2O and CO2.
Thus, it would seem that the presence of oxygen in a gas to be treated makes its demercuration via adsorption on a product containing copper sulfide and/or silver or silver sulfide inapplicable. Oxygen is actually described as being a regeneration, therefore mercury desorption, agent, and, at low temperature, as an oxidizing agent and deactivation agent.
Starting from this situation, a problem that arises is how to provide an improved method of purifying a gas stream containing CO2, at least 1% oxygen, and mercury so as to obtain a mercury-depleted gas.
A similar problem arises in the case of gas streams containing NOx and/or SOx, since these compounds have oxidizing properties, liable to react with the adsorbent.